A fundamental feature of nervous systems is that they provide plasticity of structure and function which allows animals to adapt to changes in their environment. We are only beginning to understand the underlying mechanisms, the limitations, and the behavioral consequences, such as learning, of naturally occurring plasticity in the brain. The song control system in the avian brain provides excellent opportunities for addressing such basic issues of behavioral neuroscience. Seasonal changes of environmental factors, such as photoperiod, have a profound effect on birds and most other animals. In songbirds seasonal changes in photoperiod elicit changes in circulating concentrations of gonadal steroid hormones, which in turn cause changes in song behavior and in morphological and physiological attributes of the neuroendocrine system that controls song. The exchange of social cues between individuals enhances the growth of the song system during the breeding season. The goal of this proposal is to use a comparative approach to explore the proximate mechanisms and behavioral functions of seasonal plasticity in the avian song control system. We will determine whether the social enhancement of the seasonal growth of the song nuclei is mediated by auditory stimuli. We will examine whether social stimulation from other birds enhances the growth of the song nuclei by increasing the recruitment and/or survival of newly generated neurons in the adult bird's song system. To test the hypothesis that maintenance of seasonally grown song nuclei depends on innervation from afferent nuclei, we will lesion nucleus HVc unilaterally in canaries that have been on breeding photoperiods for one month and compare the morphology of the afferent nuclei RA and Area X ipsilateral and contralateral to the lesion. The hypothesis that seasonal growth of the song nuclei is regulated by estrogenic metabolites of testosterone will be tested by measuring the morphology of song nuclei, song behavior, and plasma hormone levels in wild song sparrows implanted in early Fall with different steroids. We will use operant conditioning techniques to test the hypothesis that seasonal changes in song perception are functionally related to seasonal plasticity of the song system. The results of the proposed studies will increase our understanding of steroid hormonal and social influences on the nervous system, and of the relationship between plasticity in the adult brain and learning.